In some states, however, police officers in unmarked cars have specific rules they must follow. These differ from the rules for other police vehicles, both at the state and city levels.

"The troopers that are issued unmarked cars are given extra instructions on just that thing," a veteran trooper with the Iowa State Patrol told USA Today. Such special restrictions may include only using the unmarked cars during daylight hours.

Other states like New York place few, if any, restrictions on the use of unmarked police cars. States like Ohio, on the other hand, require all police vehicles to be "marked in some distinctive manner" and equipped with a flashing or rotating colored light.

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The use of unmarked cars for traffic stops has produced unintended, and quite tragic, results. A number of cases have cropped up of criminals posing as policemen in unmarked cars.